Enhanced generation of hydroxyl radicals

ABSTRACT

This invention provides a process for enhancing the generation of hydroxyl radicals in aqueous mixtures containing hydrogen peroxide, which process comprises supplying oxygen and magnesium oxide to the mixture and irradiating it with UV light. The process can be used, for example, for processing ballast water, industrial waste waters, and municipal waste waters.

FIELD OF THE INVENTION

The present invention relates to a method for enhancing the formation ofhydroxyl radicals in water environment in the presence of hydrogenperoxide, by the combined effect of UV irradiation, dissolved oxygen,and magnesium oxide as a catalyst.

BACKGROUND OF THE INVENTION

Hydroxyl radical (HO*) is one of the strongest oxidants that isavailable in the natural world and in the chemist's tool box. Thisspecies is extremely reactive and degrades promptly any availableorganic molecule. Hydroxyl radicals, likewise other reactive oxygenspecies, are also a very potent biocide which vigorously attackmicroorganisms and tissues, damaging cell membrane lipids, proteins,carbohydrates and DNA. The hydroxyl radical reactions are extremelyfast, having rate constants of from 10⁷ to 10⁹ M⁻¹ sec⁻¹. Consequentlythe hydroxyl radical only diffuses the equivalent of 5-10 of itsmolecular diameter before it reacts [E. Cadenas: Ann. Rev. Biochem. 58(1989) 79]. Interestingly, hydroxyl radicals and other reactive/oxygenspecies were recently recognized as a cellular signaling device [E.g.,Van Breusegem et al.: Plant Sci. 161 (2001)405-414], as well as abiological weapon used by the immune system for killing bacteria[Wentworth P. et al.: Science 298 (2002) 2195-9].

Hydroxyl radicals injure cell membrane lipids via the following chainmechanism:

a) Detachment of a hydrogen radical randomly from a fatty acid, leadingto the formation of an alkyl radical:RH+HO*→R*+H₂O

b) The alkyl radical reacts instantly with oxygen (which is available inaerobic environment) to yield an alkylhydroperoxide derivative:R*+O₂→ROO*

c) The alkyl hydroperoxide reacts with water to generate an alcohol anda hydroperoxyl radical:ROO*+H₂O→ROH+HOO*

The transformation of a fatty acid skeleton into an alcohol renders thedestruction of the membrane structure and consequently the collapse ofthe cell wall and the death of the organism.

d) The hydroperoxyl radical absorbs a hydrogen radical from anotherfatty acid molecule to yield an alkyl radical that subsequently reactswith oxygen, as shown in step b):RH+HOO*→R*+H₂O₂

The hydrogen peroxide formed becomes the source of additional hydroxylradicals which renew the cycle. This chain process propagates until atermination step such as the following one takes place:2ROO*→non-radical oxygenated products.

A fundamental feature of hydroxyl radicals (and other reactive oxygenspecies) is the benign nature of their decomposition products. Nochemicals are formed as a result of their application in various watertreatments but water.

Several methods for the generation of hydroxyl radicals are known, basedmainly on hydrogen peroxide as the source. The simplest and the mostaccepted methodology is based on the Fenton reaction where ferrous ionreacts with hydrogen peroxide in an acidic media as follows [Legrini O.:Chem. Rev. 93 (1993) 671-98]:Fe⁺²+H₂O₂→Fe⁺³+HO*+OH⁻

This reaction is made catalytic predominantly under the effect of UVirradiation of the wavelength greater than 300 nm, which allows therecycling of Fe⁺³ to the original Fe⁺², which process is named“Photo-Fenton” reaction. The main drawback of the Fenton type proceduresis their strong dependence on pH. Thus the above reaction occurs onlyunder acidic conditions, preferably at pH=2.5-3.0.[Andreozzi R. et al.:Catalysis Today 53 (1999) 51-9]. Consequently the Fenton scheme is notpracticable under the numerous situations where acidic media cannot beused, such as in ballast water treatment, for example.

Another procedure for the formation of hydroxyl, and hydroperoxyl,radicals uses photocatalysis together with semi-conductors such as TiO₂.Upon irradiation of TiO₂ particle by the light having wavelength about315-395 nm, an electron is ejected, generating a “hole” on the particlesurface:TiO₂+hν→TiO₂(h⁺)+e⁻The “hole” interacts with water to produce a hydroxyl radical:TiO₂(h⁺)+H₂O→TiO₂+HO*+H⁺

The free electron reacts with oxygen molecule to produce an anion ofsuperoxide radical which is protonated to yield a hydroperoxyl radical:e⁻+O₂→O₂ ⁻*O₂ ⁻*+H⁺→HOO*

Another widespread technique for the making of hydroxyl radical isdirect photolysis of hydrogen peroxide under UV irradiation:H₂O₂+hν→2 HO*

This procedure suffers from low efficacy due to the very low molarextinction coefficient of the hydrogen peroxide molecule (about 19dm³.mol⁻¹.cm⁻¹ at 254 nm at neutral pH [Baxendale J. H.: Trans. FaradaySoc. 53 (1957) 344].

Using EPR techniques, Giamello E. et al. [J. Phys. Chem. 97(1993)5735-40] showed that reactive oxygen species were formed upon contact ofhydrogen peroxide with the surface of magnesium oxide in an aqueousenvironment. These authors also observed a highly unexpected stabilityof hydroxyl radicals adsorbed on the MgO surface, wherein the radicalswere stable at temperatures as high as 200° C.

It is an object of this invention to provide a high efficacy method forproducing hydroxyl radicals in aqueous environment using UV irradiationof hydrogen peroxide solutions.

It is another object of this invention to provide a method for producinghydroxyl radicals in water solutions of hydrogen peroxide and oxygen,combining UV irradiation with a catalytic effect of magnesium oxide.

Other objects and advantages of present invention will appear asdescription proceeds.

SUMMARY OF THE INVENTION

This invention provides a method for enhancing the generation ofhydroxyl radicals in aqueous mixtures containing hydrogen peroxidecomprising: i) supplying oxygen or air to said mixture; ii) supplyingmagnesium oxide to said mixture; iii) irradiating said mixture with UVlight, preferably having a wavelength of from 190 to 390 nm; and iv)mixing said mixture. Said aqueous mixture is an aqueous solution orsuspension. The initial concentration of hydrogen peroxide is from 2 ppmto 250 ppm, preferably 10 ppm to 50 ppm. Magnesium oxide is preferablyadded to the mixture in one portion of from 2 ppm to 250 ppm, and morepreferably from 10 ppm to 50 ppm. Oxygen is supplied as air or pureoxygen, preferably to saturation.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other characteristics and advantages of the invention willbe more readily apparent through the following examples, and withreference to the appended drawings, wherein:

FIG. 1. Shows the effect of individual factors on the formation ofhydroxyl radicals;

FIG. 2. is a HPLC chromatogram of salicylic and hydroxysalicylic acid;and

FIG. 3. is a table summarizing conversion rates of salicylic acid undervarious conditions.

DETAILED DESCRIPTION OF THE INVENTION

It has now been surprisingly found that the presence of magnesium oxidein an aqueous mixture containing hydrogen peroxide and oxygen leads toenhanced formation of hydroxyl radicals under UV irradiation. Withoutwishing to be limited by any particular theory, the inventors believethat magnesium oxide is a strong catalyst during the creation ofhydroxyl radicals in water environment provided the system is irradiatedby a UV light. This phenomenon provides particularly good results in thepresence of oxygen. The synergic combination of hydrogen peroxide,oxygen, magnesium oxide and UV irradiation provides an effective newmeans for the generation of hydroxyl radicals in water. Surprisingly,even in the absence of hydrogen peroxide, a significant amount ofhydroxyl radicals are formed in water in the presence of oxygen andmagnesium oxide. In a preferred embodiment of the present invention,hydrogen peroxide is added to an aqueous solution in which the formationof hydroxyl radicals is desired, and magnesium oxide is injected as asuspension while mixing the aqueous solution, introducing into itoxygen, and irradiating it by UV. Any commercially available UVirradiation source can be used.

In a preferred embodiment of the method according to this invention, theinitial concentration of hydrogen peroxide is from 2 ppm to 250 ppm, thesolution is saturated with oxygen, and magnesium oxide in suspension isinjected as a catalyst to a concentration from 2 ppm to 250 ppm. Oxygenis preferably introduced into the mixture by injection of oxygen or air.

In one of the preferred embodiments of the invention, the peroxideradicals serve for the treatment of ballast water. In a preferredembodiment of this invention, hydrogen peroxide is added to sea water,intended to serve as ballast water, to a concentration of from 10 to 50ppm, air or oxygen is finely dispersed to saturate the mixture, andmagnesium oxide is added to a concentration of from 10 ppm to 50 ppm.

This invention thus provides a method for enhancing the generation ofhydroxyl radicals in aqueous mixtures containing hydrogen peroxidecomprising i) supplying oxygen to said mixture; ii) supplying magnesiumoxide to said mixture; iii) irradiating said mixture with UV light; andiv) mixing said mixture for a period sufficient to generate the desiredamount of radicals, wherein said desired amount of radicals may be anamount sufficient to reach a required effect in the mixture, for examplea biocidal effect. Said desired amount of radicals may be a requiredpredetermined quantity. Hydroxyl radicals may be quantified by physicalor chemical methods known in the art, such as measuring redoxpotentials, or reacting hydroxyl radicals with a trapping agent such assalicylic acid, etc. Said aqueous mixture can be a solution orsuspension. Air or oxygen is supplied in any suitable manner that caninject and homogenize it into the aqueous solution, preferably providingsaturation. According to a preferred embodiment of the invention the UVradiation has a wavelength of from 190 to 390 nm. Magnesium oxide ispreferably added to the mixture in one portion. The period that enablesthe generation of required amount of radicals depends on the type ofapplication of said mixture, and may be typically from few seconds toseveral hours, for example from 3 seconds to 5 hours, and preferably itis from 30 seconds to 100 minutes. The mixture may be irradiatedessentially during the whole period that is set for the generation ofhydroxyl radicals, and also oxygen may be supplied to the mixture duringthe whole of said period, or, alternatively, the application of UV andsupplying oxygen may last only a few seconds, if the required effect isattained that quickly in said type of application.

In a preferred embodiment of this invention, the pH of the aqueoussolution, in which the formation of hydroxyl radicals is desired, has avalue of from 5 to 10, and preferably from 7.2 to 9.7.

The described process can be used whenever the increased concentrationof hydroxyl radicals is desirable, an example being processing ballastwater, industrial waste waters, municipal waste waters, etc. The methodof this invention may be thus used, for example, for the purification ofsea water or municipal effluent water, however without being limited tothese examples.

The invention will be further described and illustrated by the followingexamples.

EXAMPLES Quantifying Hydroxyl Radicals

The formation of hydroxyl radicals was quantified by reacting theradicals with salicylic acid [H. Kaur et al.: Methods in Enzymology 233(1994) 67-82] to form 2,3-dihydroxybenzoic acid and 2,5-dihydroxybenzoicacid, and by detecting these products by HPLC with a UV detector (SeeFIG. 2), using column RP-18, 80% phosphate buffer pH 6.5+20% methanol,flow rate 1 ml/min, and λ_(max) 300 nm.

The degree and rate of the conversion to dihydroxybenzoic acid isproportional to the amount of hydroxyl radicals generated in a givensystem.

Example 1

Reaction was carried out in a one liter beaker equipped with a stirrer,into which a medium pressure mercury UV lamp in a quartz sleeve,Immersion Lamp TQ 150W, was immersed. Approximately half the lamp wasimmersed. Tap water passed continuously through quartz jacket to coolit. The total volume of reaction mixture was 800 ml. Illuminated lengthwas 41 mm. Salicylic acid (Fluka, purity>99.5%), 500 ppm in 400 mlseawater, and 400 ml seawater were stirred with a magnetic stirrer, andthe pH of the solution was adjusted to 7.85 with 0.5% NaOH. Theirradiation continued for 90 min. Samples were withdrawn at 10, 20, 40,60 and 90 min, and analyzed by HPLC. The conversion rates werecalculated as the ratio of (OH-salicyl.)/(salicyl.+OH-salicyl.) (seeFIGS. 1, 2, and 3).

Example 2

The reaction was started as in Example 1, but 20 ppm (16 mg) ofmagnesium oxide (obtained from Aldrich, purity 98%) was added to 800 mlof the reaction mixture after the pH adjustment, and after 5 min theirradiation started, and continued for 90 min.

Example 3

The reaction was started as in Example 1, but hydrogen peroxide, 20 ppmin 400 ml seawater, was added to salicylic acid, 500 ppm in 400 ml seawater, pH was adjusted to 7.85 with 0.5% NaOH, and the mixture was thenirradiated during 90 min.

Example 4

The reaction was started as in Example 3, but 20 ppm (16 mg) ofmagnesium oxide was added to 800 ml of the reaction mixture after the pHadjustment, and only then the irradiation started.

Example 5

The reaction was started as in Example 1, but after the pH adjustment,purging of the mixture started, oxygen being fed at a rate of 50 ml/minthrough a glass tube of diameter 0.5 cm. After 10 min, the irradiationstarted, and continued for 90 min.

Example 6

The reaction was started as in Example 2, but after the addition ofmagnesium oxide, and 5 min stirring, purging of the mixture with oxygenstarted, oxygen being introduced through a glass tube of diameter 0.5cm. After 10 min, the irradiation started, and continued for 90 min.

Example 7

The reaction was started as in Example 3, but after the pH adjustment,the mixture was purged with oxygen which was introduced through a glasstube of diameter 0.5 cm. After 10 min, the irradiation started, andcontinued for 90 min.

Example 8

The reaction was started as in Example 4, but after adding magnesiumoxide and 5 min stirring, purging of the mixture with oxygen started,oxygen being introduced through a glass tube of diameter 0.5 cm. After10 min, the irradiation started, and continued for 90 min. The pH at 90min was 7.91. The initial and final temperature of the reaction mixturewere 23° C. and 25.5° C., respectively, and the initial and final redoxpotential of the reaction mixture were 180 mV and 110 mV, respectively.The superior generation of hydroxyl radicals in the system MgO/H₂O₂/O₂can be seen in FIGS. 1 and 3.

Example 9

Some of the enhanced activity, seen in the mixtures containing MgO,might be attributed to the increased basicity in the presence of solidMgO. Therefore, two reactions were performed comparing the effect of MgOwith the effect of sodium hydroxide. Two mixtures contained 250 ppmsalicylic acid, 30 ppm hydrogen peroxide, and they were irradiated byUV. The first mixture further contained 250 ppm MgO, its pH was adjustedto 7.8 at the beginning of the reaction, and rose to 9.38 after 90minutes. The second mixture did not contain MgO and its initial pH wasadjusted to 9.55 by NaOH. Magnesium oxide had stronger effect even whencompared to the effect of sodium hydroxide, pH 9.55, which furtherdemonstrates the phenomenon of catalytic properties of MgO.

While the invention has been described using some specific examples,many modifications and variations are possible. It is thereforeunderstood that the invention is not intended to be limited in any way,other than by the scope of the appended claims.

1. A method for enhancing the generation of hydroxyl radicals (OH*) inaqueous mixtures containing hydrogen peroxide, comprising i) supplyingoxygen to said mixture; ii) supplying magnesium oxide to said mixture asa catalyst; iii) irradiating said mixture with UV light; and iv) mixingsaid mixture.
 2. The method of claim 1, wherein the aqueous mixture isan aqueous solution or suspension.
 3. The method of claim 1, wherein thehydrogen peroxide has an initial concentration of from 2 to 250 ppm. 4.The method of claim 1, wherein the oxygen is supplied by injecting airor oxygen into the mixture.
 5. The method of claim 1, wherein the oxygenis supplied to saturation.
 6. The method of claim 1, wherein said UVradiation has wavelength of from 190 to 390 nm.
 7. The method of claim1, wherein the magnesium oxide is added to the mixture to aconcentration of from 2 ppm to 250 ppm.
 8. The method of claim 1,wherein the initial concentration of hydrogen peroxide is from 10 to 50ppm, and the initial concentration of magnesium oxide is from 10 to 50ppm.
 9. The method of claim 1, wherein the pH of said mixture has avalue of from 5 to
 10. 10. The method of claim 9, wherein said pH has avalue of 7.2 to 9.7.
 11. The method of claim 1, wherein said mixing iscarried out for a period of time sufficient to generate the desiredamount of radicals.
 12. The method of claim 11, wherein said desiredamount of radicals is an amount sufficient to reach a required biocidaleffect in the mixture.
 13. The method of claim 11, wherein said periodlasts from 3 seconds to 5 hours.
 14. The method of claim 13, whereinsaid period lasts from 30 second to 100 minutes.
 15. The method of claim11, wherein said period lasts more than 5 hours.
 16. The method of claim11, wherein said desired amount of radicals is a predetermined quantity.17. The method of claim 11, wherein generated radicals are quantified bya physical or chemical method.
 18. The method of claim 17, wherein saidchemical method comprises reacting the hydroxyl radicals with salicylicacid.